The most important thing to think about when selecting an antenna is what frequencies do you listen to. If you only need VHF-Hi (think 140 - 160 MHz or so) you'll waste your money on an antenna that covers 25 - 1300 MHz. The single band antenna will provide you with gain (and possibly a lower price) while the wide band antenna provides you with no gain and a lot of frequency coverage you don't need (and if there are interfering signals within that range, coverage you don't want).
If you need good coverage on VHF-Hi and UHF (think 350 - 500 MHz) a dual-band ham antenna will give you good gain for a reasonable price. If you need 800 MHz coverage (think 750 - 950 MHz here) a good 800 MHz antenna will give you gain here for a reasonable price. If you need VHF-Low (think 30 - 50 MHz here) there are good antennas for that band as well. If you want coverage for all bands, you're stuck with zero or low gain antennas. The choice is yours as to what is best for what you listen to.
Remember that it isn't just the antenna, but the antenna system (antenna, feed line or coax, etc.) - basically everything beyond the antenna jack on your scanner - that's important. A great antenna using poor coax will give poor results. A poor antenna with the best coax available will also give poor results. A great antenna with the best coax available installed poorly will give mixed results at best.
Also remember that you often don't need the absolute best of everything. Your scanner needs just enough signal that's above the noise level for you to get good scanning. Too little and you'll get just noise. Too much and you may overload the receiver and have nothing but problems.
Figure out what you want to listen to (the RR database is a good place to start -
RadioReference.com - Scanner Frequencies and Radio Frequency Reference Database), look around to see what complications you may have picking them up (are they too far away? are there things that may block the signal like hills between you and them? do you have RF signal issues at your location - nearby towers, etc.? ...), discover what band or bands you need to cover, pick an antenna optimized for that environment and band coverage.
I know of many folks that get a wide band antenna when a single or dual band antenna would work better for them. For others, they get a single band antenna, but use it for other bands where it doesn't perform nearly as well.
A couple of things to think about are:
* Some folks market antennas that are designed for one or two bands and label them as "also works on ...". Remember that "works on" doesn't say "works well on". If it doesn't list the band on the main coverage list, don't count on it working well for you on that band.
* Antenna gain is listed in two different ways, dBd and dBi. The dBd gain number is about 2.2 smaller than the dBi gain number. If they don't say which, assume dBi since the numbers are larger for the same amount of gain. That super high 5 dB gain antenna is probably the exact same antenna as that lowly 3 dB gain antenna since the one with the larger number is probably reporting it using the higher looking number.
* Antenna gain (on multi-band antennas) is often different by band it covers so make sure you compare using like values. A dual band VHF-Hi/UHF antenna with gain figures of 6.2/9.5 dB should work just as well as a VHF-Hi only antenna with a 6.2 dB gain (assuming you don't need the UHF coverage from the dual band antenna).
* More gain is often better, but not always. If you live in a high RF area (nearby transmitters - 2-way, paging, NOAA Weather, FM broadcast, TV broadcast, or even cell phone towers) it may be better to use a lower gain antenna than one with lots of gain. Also if your listening to a simulcast system, more gain may equal more simulcast distortion issues (something very bad) so less gain (or a very directional antenna) may work better). There may be filters to help block some if the high RF signals if you have these issues.
This is just a few of the things to think about (there are many, but this should get you started in the right direction). Don't count on folks saying "I've got this and it works great" without knowing that their situation matches yours. Don't ignore them totally either, but make sure that you match what they're praising with what you're needing. That 800 MHz antenna that works great for them may be worthless to you and your VHF-Lo band listening needs. There's also not just a single answer to what's best, only various ways to solve similar problems.
